Combined cycle power plants with integrated gasification (Integrated Gasification Combined Cycle IGCC) are normally operated with a synthetic combustible gas (syngas or MBtu-gas), which is produced by gasification of coal, biomass or other fuels. This fuel differs considerably from natural gas with regard to the gross calorific value, the density and the combustion characteristics, such as flame velocity and ignition delay time. In a gas turbine with sequential combustion or reheating for IGCC power plants, therefore, both in the fuel supply system and in the combustion chambers, essential adjustments are necessary in order to cope with these differences.
A gas turbine installation with reheating, with its essential component parts, is schematically reproduced in FIG. 1. The gas turbine installation 10 of FIG. 1 comprises a unit for compressing the inducted combustion air, which comprises a low-pressure compressor 11a and a high-pressure compressor 11b which is connected downstream. The compressed combustion air is fed to a first combustion chamber 12 where it is used partially for combusting a fuel which is fed via a first fuel feed 23. The resulting hot gas is expanded in a subsequent high-pressure turbine 13, performing work, and then fed to a second combustion chamber 14 in which the unused air portion is used for combusting a fuel which is fed via a second fuel feed 23. The hot gas which comes from the second combustion chamber 14 is expanded in a subsequent low-pressure turbine 15, performing work, and then directed through a heat recovery steam generator (HRSG) 21 where steam is produced for steam turbines (not shown) of a separate water-steam cycle. The exhaust gas 22 can then be directed to an exhaust stack. The two turbines 13 and 15 are connected via a shaft 20 to the compressors 11a, b and to a generator 16 for electric power, and drive this. Compressed air for cooling purposes can be tapped from the compressors 11a and 11b, cooled down in corresponding high-pressure or low-pressure once-through coolers (OTC) 18 and 19, and then directed to the combustion chambers 23, 24 or to the turbines 13, 15 for cooling. A comparable gas turbine installation is disclosed for example in U.S. Pat. No. 5,617,718.
In the second combustion chamber of the sequential combustion the fuel is injected into the hot gas flow by a fuel lance, the shape of which is indicated in FIG. 2 of U.S. Pat. No. 5,617,718, and the construction of which is shown in detail for example in EP-A2-0 638 769. If different types of syngas are compared with natural gas, it becomes apparent that for the syngas, depending upon type and source, a larger flow cross section is required, which can be larger by the factor of 3 to 9 than the flow cross section for natural gas. At present, it is a great challenge to inject the large volumetric flows which are associated with it in the case of syngas through the fuel lance into the combustion chamber. It is theoretically possible to increase the lance diameter in order to create the necessary additional space. This, however, would have a significant influence on the aerodynamics of the burner and would entail a new construction of the combustion chamber and of the casing of the gas turbine. It is therefore desirable to keep the outside diameter of the fuel lance at the transition from natural gas to syngas constant. On the other hand, some modifications are necessary in order to reduce the residence time of the syngas inside the mixing zone of the burner and so to avoid a flashback.